1. Field of the Invention
This invention relates to extraction of geothermal heat energy from the earth and more particularly to the extraction of power from the liquid and the vapor portions of the geothermal fluid, and systems for converting the heat carried by a working fluid into electrical power.
It is known that the interior of the earth is a molten mass of rocks and is very hot. This geothermal heat energy may advantageously and efficiently be employed as a primary source of energy for the generation of power by utilizing a geothermal fluid as a carrier. The fluid may be steam released from volcanic areas or hot water which is present in volcanic and in deep alluvial deposits that are porous enough to permit percolation of water to the deep hot zones. This water may have a temperature as high as 700.degree. F. at a depth of 5,000 feet.
Additionally, the carrier fluid may be injected into the earth by means of a well where the carrier fluid is vaporized upon contact with the heat energy in the earth. This heat energy may be present in dry rock beds or in fluids, such as water, in the well.
2. Description of the Prior Art
In some areas the heat energy within the earth is carried to the surface by steam. This steam may be used directly in turbines to drive generators to generate electricity as the means of efficiently and effectively recovering the heat energy. Where there is a mixture of steam, non-condensables, such as carbon dioxide and hot water, the steam and non-condensables may be separated in a flash chamber and then used in a steam turbine. The fluid from a geothermal well may flow naturally to the surface or may have to be pumped to the surface.
An efficient way of utilizing hot fluid which may contain salts and other dissolved minerals, especially hot fluids from a geothermal well, is by passing a working fluid in heat exchange relationship through a surface exchanger with a working fluid. The working fluid has a boiling point that is below the input temperature of the hot fluid. This method and apparatus for utilizing the hot fluid as a primary source of energy represents an efficient way for generating power from hot fluids.
However, there are some hot fluids that contain such a high degree of salts and other dissolved minerals that the above method may not be efficiently and economically useful over a long period of time. For example, it is known that the water from some of the geothermal wells in the Imperial Valley in California have more than 400,000 parts per million of dissolved solids which include salts and other minerals. The passage of this brine through a surface heat exchanger can result in the deposit of silica or other types of scale therein with a substantial decrease in the heat transfer efficiency within the heat exchanger and an increase in the pressure drop through the exchanger.
A second way of utilizing the geothermal heat energy is by passing a high boiling point heat transfer fluid in direct heat transfer relation with the hot geothermal fluid. The heat transfer fluid is then cooled by vaporizing a working fluid which is allowed to expand and to produce power. This process requires two heat exchange operations, which may degrade the efficiency of the power conversion process and may increase the cost of the production of power.
A third way of utilizing the geothermal heat energy is by passing the geothermal fluid in direct heat transfer relationship with a working fluid, where the working fluid is allowed to boil and vaporize in the direct contact heat exchanger. The vaporized working fluid is expanded to produce power, condensed and returned to the direct contact heat exchanger. This process may be limited in flexibility and may not be suitable for the high temperature geothermal resources without process improvements.